In the United States, rootworms and cucumber beetles cause considerable damage to corn, soybeans, cucurbits (e.g. squashes, melons, gourds, pumpkins) and other crops. R. L. Metcalf and W. H. Luckman, Introduction to Insect Pest Management, 3rd Ed., p. 328 (1994). Control practices over the last several decades have largely relied on heavy applications of soil and foliage insecticides. Unfortunately, the application of large amounts of insecticides has led to resistance to the compounds in target insects, accelerated microbial degradation, and the widespread pollution of groundwater due to pesticide run-off. Id. Additionally, there has been a progression in usage from the relatively inexpensive organochlorine insecticides to the increasingly expensive organophosphate, carbamate, and pyrethyroid insecticides. Thus, the cost of pest management has increased dramatically. Id.
In recent years, one strategy for combatting insect infestation has been the use of insect attractants, arrestants and phagostimulants in combination with smaller quantities of insecticides. Generally, this strategy involves the use of a formulation containing one or more insect-specific phagostimulants or attractants, along with a low concentration of a pesticide toxic to that insect. After application of the formulation to crops targeted by the insect, the insects are attracted to the formulation and induced to feed, but are then killed by the insecticide.
This strategy has particular potential in the management of Diabroticite rootworms and related species. There are more than 1400 species of Diabroticites worldwide, many of which are serious pests of corn, peanuts, cucurbits, and sugarbeets in North and South America, Africa, Asia and Australia. These insects have co-evolved with plants of the family Cucurbiticae and have developed specific host-plant recognition signals for the toxic cucurbitacins characteristic of this family.
The cucurbitacins are a group of bitter-tasting, highly oxygenated, mainly tetracyclic, triterpenic plant substances derived from the cucurbitane skeleton. M. Miro, Phytother. Res. 8, 159-168 (1995). These compounds are present in many plants and function as an allomone to protect the plants from herbivores. R. L. Metcalf, J. Chem. Ecol. 12, 1109-1124 (1986). In addition, the cucurbitacins are known to have purgative, anti-inflammatory, anti-fungal, and anti-cancer properties. See M. Miro et al., Phytother. Res. 9, 159-68 (1994); Anticancer Agents Based on Natural Product Models pp. 247-254 (J. M. Cassady and J. D. Dourous, eds., Academic Press, New York, 1980).
Diabroticite beetles are known to compulsively consume plant tissue of the Cucurbitale order. When consumed by the beetles, the cucurbitacins are sequestered in the wings, and are thereby believed to act as predation deterrents. In addition to their properties as diabroticite phagostimulants, cucurbitacins also function as anti-feedants for other families of insects, and for other herbivorous pests such as birds. Cucurbitacins are among the most bitter compounds known, and in nanogram quantities deter most non-Diabrotic herbivores. Additionally, high concentrations of cucurbitacins are toxic to certain insect, bird and mammalian species.
Commercialization of cucurbitacin-based products has been very limited because (a) the triterpenes are secreted in very small quantities in cucurbitaceous plants, and (b) the existing procedures involved in obtaining pure cucurbitacins is lengthy and burdensome. See, F. T. Halaweish and D. W. Tallamy, J. Liquid Chromatography 16, 497-511 (1993); J. Guha and S. P. Sen, Plant Biochemical Journal 2, 12-28 (1975). The difficulties involved in obtaining large quantities of cucurbitacins in the past has discouraged the serious pursuit of most potential uses of this triterpenes.
U.S. Pat. No. 5,466,460 to McMahon et al. describes controlled-release microcapsules that contain insecticides and other compounds useful in crop management. In one embodiment of the invention, the capsule wall contains an anti-feedant compound comprising cucurbitacin-containing solid particles, powder or dust. The preferred cucurbitacin-containing solids used are in the form of dried, ground, gourd roots as described in U.S. Pat. No. 4,880,624.
U.S. Pat. No. 5,484,587 to Branly et al. describes baits for diabroticite beetles containing a feeding stimulant and an insecticide. The feeding stimulant comprises plant tissue containing cucurbitacin, and more specifically comprises dried buffalo gourd root in an amount of 10-100 lb. of ground root per acre.
Based upon the above-described technology, a cucurbitacin-dependent control method for adult cucumber beetles has reportedly been developed. By lacing cucurbitacin-containing plant tissue with insecticide, the beetles are "tricked" by the feeding-stimulant cucurbitacins into eating the toxins. This method is pest-specific and constitutes a point-source or broadcasted bait capable of killing 99% of the beetles consuming it. See, R. L. Metcalf et al., J. Econ. Entomol. 80, 870-875 (1987); D. R. Lance and G. R. Sutter, J. Econ. Entomol. 83, 1085-1090 (1990); J. Econ. Entomol. 84, 1861-1868 (1991). Additionally, the method uses as little as less than 10% of the insecticide quantity per unit area required for conventional insect control. However, the method has achieved only limited success due to the use of ground root tissue as the cucurbitacin source, in that it appears that this form of cucurbitacin is effective in the control of adult beetles only, while much of the damage caused by the diabroticite beetles is caused by the larval form. See, B. P. Spike and J. J. Tollefson, J. Econ. Entomol. 84, 1585-1590.
The use of purified cucurbitacins in place of plant tissue is one possible solution to this problem, as purified cucurbitacin has been shown to be an effective feeding stimulant for diabroticite beetles in both adult and larval forms. Unfortunately, there are no products marketed today that utilize purified cucurbitacins. It would therefore be desirable to provide a simple, cost-effective and high-yield process for isolating and purifying cucurbitacin compounds in large quantities. It would also be desirable to use the cucurbitacins isolated in this manner in methods and formulations for the control of insects, particularly diabroticite beetles.